From what I understand the grp engines and the old ninjas (not the os based jx series) the parts are interchangeable as the the older ninjas were made by grp. The older ninjas are MR ninja with the MR. Meaning Mario Rossi tuned. Just like the new grps
Hope this helps, as far as I know there is not another rod that fits the grp besides the nontuned version rod (they are also hard to find))